Positioning or navigation technology has the purpose of determining a geographic position of an object, equipment or a person carrying the equipment. The position is typically given with respect to a specified coordinate system. Such positioning has become more and more interesting in many fields of applications during recent years. One approach to solve the positioning is to use signals emitted from satellites to determine a position. Well-known examples of such systems are the Global Positioning System (GPS) and the coming GALILEO system. The position is given with respect to a specified coordinate system as a triangulation/trilateration based on a plurality of received satellite signals.
Assisted GPS (AGPS) is an enhancement of the GPS system, to facilitate integration of GPS receivers into e.g. mobile terminals of cellular and/or cellular communication systems. The GPS reference receivers attached to a cellular communication system collect assistance data that, when transmitted to GPS receivers in terminals, enhance the performance of the GPS terminal receivers. Additional assistance data is collected from the cellular communication system directly, typically to obtain a rough initial estimate of the position of the terminal together with a corresponding uncertainty of the initial estimate. This position is often given by a so called cell identity positioning step, i.e. the position of the terminal is determined with cell granularity.
Fine time assistance means that the GPS receiver is provided with highly accurate information or data related to a satellite time reference, e.g. the global GPS time, and satellite positions in space. This, in turn, allows upper and lower bounds of a search window for the code phases of signals transmitted from all GPS satellites to be computed for terminals that reside anywhere in a region obtained by an initial, relatively inaccurate positioning step. This follows since the times of transmission of the signals from the GPS satellites are synchronized with extreme precision, and since the orbits of these satellites can be calculated in the cellular communication system using other types of assistance data obtained from e.g. GPS reference receivers.
There are two main sources of errors present in this process. The first is caused by the fact that the initial position of the user equipment is normally not known with better accuracy than the size of the cell to which it is connected. The second main error contribution is caused by the distribution of GPS time to the terminal. Both sources of errors manifest themselves in an uncertainty of the exact location of the code/Doppler search window of the GPS receiver of the user equipment that is used to lock onto the ranging signal of one specific space vehicle (SV). In WO06001738, methods and means for handling the search window uncertainty caused by the initial position uncertainty are disclosed. However, the determination and generation of the exact relation between a satellite reference time, e.g. the GPS Time Of Week (TOW), and the timing of the cellular frame structure is not discussed.
The establishment of the time relation between GPS TOW and the timing of the cellular frame structure can be performed with two main methods. One way is to have dedicated reference GPS receivers in each radio base station that time stamps the cellular frame boundaries of the uplink and downlink connections. However, such solution calls for relatively expensive additional hardware in the radio base stations and have furthermore redundancy drawbacks.
Another approach is to utilize measurements from user equipments of opportunity, or from dedicated measurements, performed in A-GPS capable user equipments. In the case of user equipment based A-GPS, these user equipments establish GPS TOW and can hence perform the time stamping, whereas a more complicated procedure has to be used for user equipment assisted GPS. There, the sought relation is signaled to the positioning node in the network.
The generation of basic measurements resulting in the fine time assistance data is performed by the user equipment, which measures the number of chips to a cellular frame boundary, at a pre-determined GPS time. However, the establishment of GPS time in the user equipment is non-trivial, in particular in user equipment assisted GPS, since there in prior art are ambiguous ways of interpreting the transmitted information.
If the GPS time in some user equipments is established correctly, the results of the measurements are reported to the positioning node. In a WCDMA case, the positioning node is comprised in the RNC and the report takes place over the RRC protocol as a GPS Timing of Cell Frame measurement. The positioning node collects the measurements and establishes the relation between GPS TOW and the cellular frame timing, e.g. the UTRAN frame timing, for each cell of the cellular system. This collection is not trivial either, also due to possibilities for the ambiguous interpretation mentioned above.
When another user equipment is to be positioned making use of AGPS, the positioning node prepares assistance data, and in particular fine time assistance data. The data comprises an expected relation between the cellular frame timing, (e.g. UTRAN timing) and GPS time (TOW). This data is related to a specific pre-determined reference point, preferably located at the center of the cell. The data furthermore comprises expected code phase and Doppler shift at the reference point. The uncertainty of the expected code phase and Doppler imposed by the spatial extension of the cell may also be provided, see e.g. WO06001738. This information is typically encoded by the positioning node e.g. as a recommended search window for each SV, expressed as a search window center point and a search window length. However, the expected code phase and Doppler is also influenced by the mentioned error in the relation between the UTRAN frame timing and the GPS TOW. Without any information about possible or probable uncertainties in that relation, the window for the expected code phase and Doppler to be searched has to be made unnecessarily wide in order to cover all possible cases. This in turn leads to unnecessarily large computational efforts as well as unnecessarily long processing time.